1. Field of the Invention
This invention relates generally to a planet-pinion carrier assembly for a planetary gear system to be used in an automatic transmission and, more specifically, to the planet-pinion carrier assembly, wherein thrust washers disposed between axial ends of each planet pinion and a planet-pinion carrier rotatably supporting the planet pinions have a surface hardness large enough to ensure improved durability and reliability.
2. Description of the Background Art
There is known a planet-pinion carrier assembly for a planetary gear system to be used in an automatic transmission, such as disclosed in Japanese First (Unexamined) Utility Model Publication No. 62-40346.
In this known structure, the carrier assembly includes a planet-pinion carrier having a rotation axis, planet pinions each rotatably mounted on a pinion shaft which is fixed to the pinion carrier, a needle roller bearing disposed between the pinion shaft and the planet pinion for rotatably supporting the planet pinion, and thrust washers. Specifically, the pinion carrier has first and second sections extending radially with respect to the rotation axis and axially spaced apart by a predetermined distance from each other for receiving the planet pinions therebetween. The first and second sections are fixedly connected to each other and fixedly support each of the pinion shafts therebetween. The planet pinion is rotatably mounted on each pinion shaft through the needle roller bearing. Further, a pair of the thrust washers are disposed in slidable contact with each other between one axial end surface of each planet pinion and the first section, and another pair of the thrust washers are disposed in slidable contact with each other between the other axial end surface of each planet pinion and the second section, for supporting axial loads applied thereto from the planet pinion and the needle roller bearing. By utilizing two thrust washers arranged in slidable contact with each other, a relative rotational speed between the two thrust washers, when the planet pinions are rotated, can be reduced so as to improve the durability of the thrust washers.
In this known planet-pinion carrier assembly, however, a problem exists that the durability of the thrust washers is insufficient when the applied load is large, i.e. when the planet pinions are rotated at a high speed and with a large torque. Specifically, in the known structure, each thrust washer is formed by applying a soft nitriding process to a high carbon tool steel to have a surface hardness of about Hv (Vickers hardness) 400. Accordingly, the thrust washers, particularly those which face the axial end surfaces of each planet pinion are subject to abrasion when the applied load is large, due to the contact with axial ends of the needle roller bearing and due to the load applied unevenly to the thrust washers which is caused by inclination of each mounted planet pinion.